Papillomaviruses (PVs) infect the epithelia of animals and man where they generally induce benign proliferation at the site of infection. However, there is a strong association between malignant progression of human genital lesions and certain HPV types, most frequently HPV16. We have generated virus-like particles (VLPs) for HPV16 and other PVs that consist of the L1 major capsid protein or L1 plus L2, the minor capsid protein. To explore the possibility that these reagents could potentially serve as the basis for a subunit vaccine to prevent PV infection, we have conducted prophylactic vaccine trials in animal models. As a follow up to a previous study in which we demonstrated that cottontail rabbit (CR)PV VLPs protected rabbits from cutaneous challenge with infectious CRPV, we have conducted a similar trial in a mucosal model. Systemic inoculation of bovine (B)PV type 4 VLPs protected cattle from oral mucosal challenge with infectious BPV4. We have developed two assays that should be useful for quantitatively evaluating neutralizing antibody titers in human VLP-based vaccine trials. One is an in vitro neutralizing assay for HPV16 that is based on infectious virus consisting of the HPV16 capsid proteins and the BPV1 genome. The second is a VLP hemagglutination inhibition assay which, in the animal PV studies, correlated well with in vitro viral neutralization and in vivo protection from experimental infection. Comparison of the activities of rabbit sera raised against VLPs of seven genital HPV types in the two assays indicated that protection elicited by VLP-based vaccines in humans is likely to be type specific. We have obtained strong evidence that our recently developed VLP-based ELISA measures past as well as current genital HPV infection. Seroprevalence in the ELISA correlated with cervical cancer risk, even in cohorts where current genital HPV DNA status did not. In a series of studies to quantitatively assess the risk associated with HPV16 infection for various human cancers, we and our collaborators have found a strong association between the presence of HPV16 virion antibodies and cervical, vulvar, anal, and (unexpectedly) esophageal cancer but not penile cancer, in cross-sectional case-control studies. In prospective studies, seropositivity was strongly associated with the subsequent development of cervical and esophageal, but not laryngeal or oral.